The dimorphic blood picture in sideroblastic anemia suggests the coexistence of at least two types of erythrocyte, the populations with the smaller RBCs being responsible for the crucial abnormality, namely the impairment of iron incorporation into protoporphyrin. In a patient with sideroblastic anemia who subsequently developed acute myelocytic leukemia, we found a light and a heavy cell population to be morphologically different, the lighter cells showing a smaller MCV and lower MCH, a higher reticulocyte count, fluorocyte count and protoporphyrin content. Radioiron uptake and 51Cr survival were 3.5% and 8.2 days for the lighter fraction, 37% and 20 days for the heavier fraction. Thus, the impairment of iron incorporation, the high erythrocyte protoporphyrin content, and the reduced survival of red cells in this patient with sideroblastic anemia appeared to be largely confined to the red cell population with the smaller RBCs.

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